The present invention relates to an outlet valve lifter for an internal combustion engine whereby the outlet valve lifter is arranged between an outlet cam and a valve push rod connected to an outlet valve and actuated by an external pressure source supplying a pressure medium.
In order to increase the braking power of internal combustion engines in commercial vehicles, it has been known to open the outlet valve during the compression cycle shortly before the ignition top dead center position is reached in order to release the air and thereby prevent back expansion. For opening the outlet valve before the ignition top dead center position is reached, a hydraulically actuated valve push rod is arranged between the outlet cam and the outlet valve which is actuatable by an external pressure source. Between the outer portion of the outlet valve lifter and an inner portion of the outlet valve lifter which is slidable inside the outer portion, a pressure chamber is provided which is connected via a bore to the pressure source. The pressure source is commonly in the form of a hydraulic pump unit. When it is desired to lift the outlet valve before the ignition top dead center position, the hydraulic pump unit is actuated and the pressure chamber is filled with the pressure medium so that the inner portion will lift the outlet valve via the valve push rod against the force of the valve spring, whereby the outlet valve lifter in this phase rests at the base circle of the cam shaft. The cam of the cam shaft only serves to lift the outlet valve during the outlet cycle. In the breaking operation of the engine, the outlet valve opens during the course of the compression cycle. Due to the increasing pressure during the compression cycle, the force that is exerted onto the outlet valve also increases. This force is transmitted via the valve push rod to the inner portion of the outlet valve lifter and results in a pressure increase which is substantially greater than the pressure required for opening the outlet valve against the force of the vaIve spring. This pressure is transmitted via all of the pressurized parts to the external pressure source and results in damage by cavitation, erosion, and leakage.
It is therefore an object of the present invention to prevent such detrimental increased pressures outside the valve lifter in order to expose all of the pressurized parts only to the relatively low pressure required for the additional lifting of the outlet valve so that damages are essentially prevented and a high operational safety over an extended service life is ensured.